This invention pertains in general to semiactive radar guidance systems for guided missiles, and in particular to circuitry to compensate for the effects of vibration-induced noise in such guidance systems.
As is known, a semiactive radar seeker in a guided missile employs a so-called “rear receiver” to provide a coherent reference signal for Doppler processing of the target return signal received by the “front” receiver in such a seeker. That is to say, a rear receiver is arranged to respond to signals transmitted from a control radar to provide a coherent local oscillator (LO) signal for the first downconversion mixers in the front receiver.
The frequency of the signal out of the local oscillator is controlled by means of an automatic frequency control/automatic phase control (AFC/APC) tracking loop, referred to hereinafter as a quadricorrelator and described in U.S. Pat. No. 4,228,434 entitled “Radar Receiver Local Oscillator Control Circuit,” inventors Williamson et al, issued Oct. 14, 1980 and assigned to the same assignee as the present invention.
It has been determined that vibration-induced noise in the circuitry may be effective to cause the quadricorrelator to switch between either one of two stable states when vibration-induced phase error exceeds the dynamic range of the quadricorrelator. When such switching occurs, a concomitant 180° phase change in the local oscillator signal for the front receiver also occurs with the result that, for a finite period of time, tracking of a target is not possible.